Cellular systems utilize a plurality of base stations to connect user terminals, such as cellular telephones, to the cellular system. Each of the user terminals initially communicates with a base station to exchange information with the system. Communications from the user terminals to a base station are called uplink (“UL”) communications, and communications from the base station to the user terminals are called downlink (“DL”) communications.
Current Institute of Electrical and Electronics Engineers, Inc. (“IEEE”) 802.16e Worldwide Interoperability for Microwave Access (“WiMAX”) and IEEE 802.16-2004 (also known as IEEE 802.16d) Orthogonal Frequency Division Multiplexing (“OFDM”) fixed broadband wireless access only networks in Time Division Duplex (“TDD”) mode are configured to have fixed allocations of the DL and UL portions, but nothing prevents them from being dynamically adjusted. A fixed DL and UL allocation in an OFDM system or an Orthogonal Frequency Division Multiple Access (“OFDMA”) system, however, may cause some cell sites to be under-utilized while other cell sites are congested. Accordingly, system performance may become adversely affected, degrading the servicing of calls and possibly resulting in dropped calls.
The reason why fixed DL and UL allocations are utilized is due to the limitation in synchronizing DL and UL operations with other nearby cell sites. Specifically, when DL and UL transmissions are not synchronized, interference results, and system operation is adversely affected. Because DL transmissions are typically much stronger than UL transmissions, there is a strong possibility that some, or all, of the UL transmissions that occur when neighboring base stations are making DL transmissions are not properly received by the intended base stations due to this interference. As a consequence, all base stations in the same band typically must have their TDD transmit cycles synchronized. This usually occurs by allocating the fixed DL and UL ratio which may, however, cause some cell sites to be under-utilized while certain other cell sites are congested.
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